The ParentNode.childElementCount read-only property returns an unsigned long representing the number of child elements of the given element.

This property was initially defined in the ElementTraversal pure interface. As this interface contained two distinct set of properties, one aimed at Node that have children, one at those that are children, they have been moved into two separate pure interfaces, ParentNode and ChildNode. In this case, childElementCount moved to ParentNode. This is a fairly technical change that shouldn't affect compatibility.